Rectal Leakage

I have to always wipe my rectal area after going to the bathroom to pee and there is leakage there that smells bad as if I had released a bowel movement but did not. It is not like a mucus but it's like if I did not wipe my self completely after a bowel movement there is always something there. What could this be? I have to get a colonoscopy done because my sister who is 43 has colon cancer and I am 37. Would they be able to find out anything or what kind of doctor would I go to??